The Hammersmith Flyover partially reopened today, following a three-week closure due to corroded support cables. But what exactly went wrong, and could it happen elsewhere? We don't have the answers, but we asked a Structural Engineering PhD to try and explain the corrosion, and what it might mean for the future of the hashtag #jammersmith.
Ever since work began on the Cromwell Road Extension in the 1940s, a complicated set of junctions at Hammersmith Broadway prevented the scheme from linking up with the Great Western Road. In 1962 the Hammersmith Flyover opened to much acclaim, providing this missing link. It is this structure whose current state of ill-health is bringing West London to a grinding halt. The source of its woes are the corroding steel reinforcement cables, but they are most certainly not to blame.
The Hammersmith Flyover is an ingenious design. It was necessary to provide an elevated carriageway 61 feet wide in a space that was only 28 feet wide using a construction site suspended above a busy arterial traffic route that was to remain operational. Conventional elevated roadways at the time were supported at either side, but adjoining property and infrastructure ruled this out at Hammersmith. The solution instead was a roadway built upwards and outwards from a narrow centrally supported 'spine', from which protrude fish bone-like cantilever beams providing the required width for the road-deck (see Figures 1 and 2). The roadway could then be laid causing no interference to the traffic below. The roadway itself also performs a structural role, known as 'diaphragm' action, transmitting lateral and twisting forces to the columns. This combination of single columns and a large, dual-purpose cantilevered superstructure had never been attempted before in the United Kingdom.
The limited space also prohibited conventional construction methods. Normally spans of this size are constructed on site, forming one large beam and then hoisted into position; this requires large casting yards that were clearly not available in Hammersmith. Instead, much smaller 'beam segments' were fabricated off-site and shipped in when they were needed (Figure 3). It is this aspect of the design that is connected to the problems being encountered today.
The principle function of the central beam is to transfer the load of the road and the vehicles above to the columns and then to the ground via the foundations. The beam achieves this primarily through a mechanical action called 'bending'. As any beam bends, the top face is compressed and the bottom face is in tension (see Figure 4). It is the job of the structure and the materials it is constructed from to resist these compression and tension forces as efficiently as possible.
The Hammersmith Flyover is a post-tensioned reinforced concrete structure. In engineering practice, concrete is considered to be a very cost-effective means of resisting the compression encountered in bending, but it is assumed to have little or no resistance to tension. To circumvent this problem, the regions known by the engineer to be experiencing tension are reinforced with steel. Normally the reinforcing steel is positioned as needed in a continuous, uninterrupted arrangement for the entire span and concrete is poured over the top, forming a complete element (see Figure 5). With the Hammersmith Flyover, this was not possible as the joints in the beam segments would break the continuity of the tensile steel and the span would simply collapse.
Instead, a method called `post-tensioning' was used, where the reinforcement bars normally used by concrete cast in-situ are replaced with cables that can be threaded longitudinally through holes in the abutting beam segments (Figures 6 and 7); the role of post tensioning in the Hammersmith flyover can be replicated with a stack of books and a helpful colleague, as shown in Figure 8. Once in place, the cables are tensioned with jacks until the bottom side of the beam is squeezed together. The cables are then anchored into position and grout is pumped into the cavities around them to provide protection against corrosion; it is these very cables that are now corroding.
It may be tempting to conclude that the post-tensioning method would constitute a design flaw, since the cables appear to be so critical and apparently vulnerable, but the Hammersmith Flyover was very much designed around this method and it has several features that were incorporated to prolong its life and ensure safety.
For a structure of this type, water and salt pose the greatest threat to the integrity of the cables, so the obvious measures to take are to ensure that water and salt do not get anywhere near them. This is achieved by two main features. Firstly, the road surface is continuous and there are no expansion joints in the post-tensioned regions (another pioneering feature of the structure is the use of a single expansion joint, with the rest of the expansion being taken by columns mounted on bearings); whilst the anchorage points are near the surface, they are still protected. Secondly, the roadway is heated (Figure 9) so that when temperatures fall below 3°C and when there is ice on the road, it switches on, removing the need to apply salt and grit. It is also apparent from design drawings that great care and effort went into designing the drainage system, with purpose-made channels that direct water into a ducting system located in the central compartment of the spine beam. The post-tensioning cables run through entirely separate and sealed compartments in the structure.
So why, despite these features, are the cables in such a poor state of repair? Without knowing exactly what the eighty or so engineers and technicians have discovered, it is impossible to be sure from a desk. However, there are a few possible avenues of failure.
Firstly, the local authority might have persisted with a salting and gritting programme despite there being no need for it — this seems all the more likely in recent years, with the visible presence of gritting machines serving as a propaganda tool for councils to say 'we're working jolly hard'. Secondly, in 2003 the original water-proofing was replaced — because of the pressure to re-open quickly, this might not have been done correctly, the new road surface might not have followed the correct profile to ensure adequate drainage, and possibly the heating system might not have been re-installed effectively. Finally, all structures need to be inspected and the Hammersmith Flyover could have been the victim of neglect. Regular inspections are especially important for modern and efficient structures. It is quite likely that inspections were originally intended to be carried out at intervals under the assumptions of (i) a 1960's traffic flow and (ii) the road not being gritted, but changes in traffic flow and possibly some ignorance on the council's part could have undermined these assumptions. This last mode of failure seems the most plausible.
The next question is, how unsafe is the Hammersmith Flyover and will it ever re-open fully? Again, without evidence, definitive answers are impossible at this stage, but the notion that the structure could collapse any minute seems a little far-fetched. The structure features considerable redundancy and factors of safety. The cables themselves are arranged in overlapping groups, passing through two spans with four separate anchorage points. It is very unlikely that all of these vulnerable locations are corroding at the same rate. Also, the cables are only subjected to 58 per-cent of their maximum allowable stress, so it would take multiple and simultaneous failures to bring the structure down.
The factors of safety involved are also considerable. Common to all structures is the requirement to assume the unloaded (or 'dead') weight of the structure is 1.5 times what it actually is. The expected applied (or 'live') load of the structure is multiplied 2.5 times. The Hammersmith Flyover goes beyond this: the normal live load at the time for four-lane elevated highway structures was to assume that the most severe load case was a simultaneous occupancy of two and two thirds of the lanes with cars. The Hammersmith Flyover assumes that all four are occupied. Even at rush hour, it is usually the case that only one carriage way is gridlocked, with free-flowing, low density traffic in the opposite direction.
So it is reasonable to say that even under normal use, the bridge probably will not fall down. Nevertheless, the problems it is facing will get worse if an adequate solution is not found. If the damage is not too extensive, structural steel, or reinforced concrete can be retro-fitted to the affected areas. If the problem spreads, then there is a chance that the bridge will need to be replaced.
So, what about other similar structures? Nearby is the Western Avenue Extension, or the `Westway'. Section Five of this scheme is very similar in design to the Hammersmith Flyover, being a centrally supported post-tensioned structure. However, this is a later design, with fewer parts and, interestingly, anchorage points for the post-tensioning cables within the central section, rather than within the structural road deck (Figures 10 and 11). This places them much further away from the damaging effects of water and salt.
The Bow Interchange, while similar in appearance to the Hammersmith Flyover, is not the same structural system, nor is the flyover at Bricklayer's Arms; these are both standard reinforced concrete structures. Nevertheless, if these structures, like any other, are not adequately inspected and maintained, they too will suffer serious damage with very similar implications for traffic, disruption and cost as the Hammersmith Flyover.
By Andrew Foster
PhD candidate (structures)
Department of Civil and Environmental Engineering
Imperial College of Science, Technology and Medicine